1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copying machine, a printer. More particularly, the invention relates to an image forming apparatus provided with fixation means for fixing images recorded on a recording material by means of heat and pressure given to it.
2. Related Background Art
Conventionally, a large number of electro-photographic copying machines, printers, or the like have adopted a heat roller method or a film heating method as fixation means, which is a contact heating type having a good thermal efficiency and safety. In recent years, fixing apparatuses of a film heating type are widely in use particularly in consideration of energy saving.
A film heating type fixing apparatus of the kind is disclosed, for example, in Japanese Patent Laid-Open Application No. 63-313182, Japanese Patent Laid-Open Application No. 2-157878, Japanese Patent Laid-Open Application Nos. 4-44075 to 4-44083, and Japanese Patent Laid-Open Application Nos. 4-204980 to 4-204984, respectively. The apparatuses thus disclosed are such that a heat resistive film is closely in contact with a heating body by means of a pressure member to convey it slidably, while a transfer material that carries images yet to be fixed is brought into a pressure contact nipping unit formed by the heating body and the pressure member, which nips the heat resistive film, and the images yet to be fixed are then fixed upon the transfer material as permanent images by means of heat and pressure given to them from the heating body and the pressure contact nipping unit through the heat resistive film.
Such a fixing apparatus of a film heating type as described above is able to use a linear heating device having a lower heat capacity as its heating body, and a thin film having a lower heat capacity as its film to be adopted in this respect, hence making it possible to save the dissipation of electric power, as well as, to shorten the waiting time (that is, to provide a quick start capability). Among the film heating type fixing apparatuses, the pressure roller driving type, which feeds and carries a fixing film and a transfer material by driving a pressure roller, is able to eliminate the provision of rollers for use of fixing film suspension and rotation, film deviation controlling mechanism, and the like. Therefore, this type has advantages in making the apparatus smaller and reducing the costs of manufacture as well.
However, the image forming apparatus, which is provided with the fixing apparatus using the conventional pressure roller driving type film heating method, tends to allow the peripheral speeds of the pressure roller to vary due to its thermal expansion caused by heat generated by the heating body, because the elastic layer of the pressure roller of the apparatus is formed by heat resistive rubber, which makes the thermal expansion of the roller greater. Then, the conveying speed of a transfer material changes considerably in the fixing position depending on whether the pressure roller is in a cooled condition or in a heated condition.
FIG. 19 is a graph which illustrates the relationship between the changes of sheet conveying speed upon fixation using ordinary sheets supplied as the transfer material, and the sheet conveying speeds in the fixing and transfer portions. A solid line indicates the sheet conveying speeds Vfu in the fixing portion. A broken line indicates the sheet conveying speeds Vtr in the transfer portion.
As shown in FIG. 19, the sheet conveying speed Vfu in the fixing portion becomes faster gradually along the thermal expansion of the pressure roller from beginning with the sheet supply while the pressure roller is in a cooled condition. The thermal expansion thereof is increased as the number of supplied sheets increase. The speed is caused to vary continuously until the thermal expansion of the pressure roller reaches its saturation. There, the changes of the sheet conveying speed in the transfer portion are different depending on the structure of the pressure roller (such as the thickness of the elastic layer, the presence and absence of the release layer for use of the surface layer release), as well as the intervals at which sheets are supplied. However, the range of fluctuation of such speed from the supply of the first sheet up to the salutation of the thermal expansion is at least approximately 1.5% of the process speed, and as large as 4% thereof if the fluctuation should be large. As described above, if the sheet conveying speeds change in the fixing position depending on the heated conditions of the pressure roller, there occurs the difference in the sheet conveying speeds between the transfer and fixation. Hence, if one sheet is present in the transfer and fixing positions at a time, there occurs an event that the sheet is pushed in or pulled between them.
If the sheet conveying speed Vfu in the fixing portion is slower than the speed Vtr in the transfer portion so that the sheet is pushed into the fixing apparatus, that is, (Vtr&gt;Vfu), the sheet is caused to sag between the transfer and fixation at P1 in FIG. 20A. Then, the difference in speeds .DELTA.V2 between the Vfu and Vtr becomes more than a specific value as indicated in the area A in FIG. 19. If this sagging becomes larger, the sheet is in contact with the surrounding structures, thus affecting the images yet to be fixed. In some cases, the so-called image scrubbing may take place as at P2 in FIG. 19.
In order to avoid such image scrubbing as this, it should be good enough if only the fixing speed Vfu for the first sheet is set more than the area A. However, if the speed Vfu in the fixing portion is set faster, the thermal expansion takes place on the pressure roller as shown in the area B in FIG. 19. Then, if the sheet conveying speed for fixation should become faster than the speed Vtr in the transfer portion, an event occurs that the sheet is pulled between the transfer and fixing portions as at P3 in FIG. 20B. As a result, the images on the trailing part of the sheet are caused to fall behind the transfer nipping, hence the density unevenness of half tone images or thicker images of characters is created in some cases.
The speed differences .DELTA.V1 and .DELTA.V2 between the transfer and fixation that may create such phenomena as described above are determined by the length of a sheet to be supplied, and the conveying distance between the transfer and fixing portions of an image forming apparatus. The longer the sub-scanning direction of a supplied sheet with respect to the length of the carrier path between the transfer and fixation, the more likely such phenomena in the smaller speed differences between .DELTA.V1 and .DELTA.V2, will take place.
In this respect, it may be possible to set a sufficient length of the carrier path between the transfer and fixation. In this case, however, the size of the image forming apparatus should be made extremely larger. This is not desirable after all.
Also, the thickness of the elastic layer of the pressure roller may be made thinner so as to suppress the thermal expansion of the pressure roller. Then, the speed changes in the fixing portion are made smaller. On this assumption, it is conceivable to set the fixing speeds within a range where no image problems may be encountered. However, if the thickness of the pressure roller is made thinner, there are some cases where no sufficient fixation is obtainable due to the inability of securing a good nipping condition needed for an intended fixation. The higher the process speeds of an image forming apparatus that particularly needs a wider fixation nipping, the more it becomes difficult to establish compatibility between the speed changes and the fixing capability.
Also, conceivably, it is made possible to prevent the sheet conveying speeds from being varied for fixation upon printing, while maintaining the pressure roller in a state that it is allowed to be sufficiently expanded thermally at all times. In this case, however, an idle rotation is needed for the pressure roller for a period of several minutes to enable its thermal expansion to reach saturation. At the same time, the pressure roller should be heated at specific intervals even when it is not engaged in printing. This heating is needed for maintaining the thermally expanded condition of the pressure roller. Then, in some cases, there may take place the inability of demonstrating the fundamental characteristics of a film heating type fixing apparatus that uses a fixing film having an extremely small heat capacity in order to make the rising time of the apparatus shorter for the suppression of electric power dissipation.